ISSN 1022-8594 2018 Jahangirnagar University Journal of Science JUJS Vol. 41, No.1, pp.87-98 Thermodynamic Studies of Some Symmetrical Electrolyte’s Solution in Aqueous-Organic Solvent Mixtures Md. Minarul Islam* and Md. Abdullah-Al-Mahmud Department of Chemistry, Jahangirnagar University, Savar, Dhaka-1342 Abstract Thermodynamic properties of the solutions of manganese sulphate (MnSO4), nickel sulphate (NiSO4), and zinc sulphate (ZnSO4) in aqueous-organic mixtures containing 5%, 10%, 15% and 20% (w/w) of ethanol have been measured at 250C in the concentration range of 2.0x10-4 to 12.0x10-4 mol dm-3. The standard Gibbs free energy, entropy and enthalpy changes were calculated from the conductance data using the values of ion-association constant by thermodynamic equations. The Gibbs free energy, entropy and enthalpy changes were investigated depending on the amount of ethanol percentage. 0 Gibbs energy of transfer (G t) from aqueous media to aqueous-organic mixtures of individual ions was also calculated in different solvent mixtures. The contributions of electrostatic and non-electrostatic part to the transfer free energy were also calculated from free energy of transfer. The values of Gibbs free energies were found negative for each electrolyte which is the sign of spontaneous association process. The values of entropies and enthalpies were found positive for each electrolyte over the entire range of solvent compositions. Keywords: Electrolytes, ion association constant, Gibbs free energy, entropy, enthalpy, Gibbs energy of transfer. Introduction Thermodynamic properties for the solutions of 2:2 electrolytes in aqueous and aqueous-organic solvent mixtures are of great interest from both theoretical and practical point of view. Researches on the association/solvation of electrolytes in mixed solvent with different compositions are of great importance for obtaining information about solute-solute and solute-solvent interactions in solutions. Knowledge of the state of association of the electrolytes and their interactions with the solvent molecules is essential for making an optimal choice of solvent and electrolyte in such systems. Manganese, iron, cobalt and copper are essential in biological systems for various enzymes[1]. Cadmium and zinc * E-mail of correspondence: [email protected] 88 Islam and Mahmud are of interest in environmental and soil science research and have been reported as major contaminants in the subsurface environments of a number of Department of Energy sites [2]. Metal salt solutions are key components in many natural and industrial procedures [3,4]. The knowledge of their properties is very crucial for a variety of chemical processes and industrial problems. The study of electrolytes solution is basically important to understand the properties and behavior of aqueous- organic mixed solvent systems. Due to the importance in biological and industrial system it is significant to measure the thermodynamic properties for these electrolytes solutions [5]. The understanding of Gibbs energy of transfer of metal ions from water to aqueous-organic solvent mixture is necessary to know the structural features of ion-ion and ion-solvent interactions. Solvent properties such as the viscosity and the dielectric is very important to take into consideration in order to determine the extent of solute-solvent interactions and the ionic association that allowed to understand the unique structure of the solvent. Ethanol is a polar solvent having lower dielectric constant than that of water. It associates in the liquid by hydrogen bonding. In this alcohol, ionic association is interpreted in terms of a multiple-step association process involving hydrogen-bonded solvation of anions [6]. In our previous paper, we have reported the equivalent conductance, limiting equivalent conductance and association constant data of manganese sulphate, nickel sulphate and zinc sulphate in ethanol-water mixtures with various solvent compositions at different temperatures [7]. Our present investigation is to estimate the thermodynamic properties based on association constant data of these electrolytes at a particular temperature with various solvent composition. In this present paper, we report the thermodynamic properties of manganese sulphate, nickel sulphate and zinc sulphate solutions in ethanol- water mixtures at 300C. The change of standard values of Gibbs energy (ΔG0), enthalpy (ΔH0) and entropy (ΔS0) are measured for various solvent compositions on the basis of ion-association constant data. The transfer of 0 Gibbs energy ( Gt ) from water to ethanol-water mixtures is also calculated to understand nature of ion-pair formation in the mixed solvent systems. Thermodynamic Studies of Some Symmetrical Electrolyte’s Solution 89 Results and Discussion The standard Gibbs energy, enthalpy and entropy of the ion-association process were calculated from the temperature dependence of the ion- 0 association constants (KA). The standard enthalpy changes (ΔH ) for each electrolyte were estimated from the slope of logKA vs. 1/T plots (Fig.1) with the help of the following equation H 0 ln K C A RT Figure 1: Plots of logKA vs. 1/T for NiSO4, MnSO4 and ZnSO4 in 5 wt.% of ethanol-water mixture. The measured values of standard enthalpy changes of NiSO4, MnSO4 and 0 ZnSO4 solution are recorded in Table 1. It shows that all the values of ΔH of each electrolyte are positive. This result indicates that the ion- association process is thermodynamically endothermic. The negative ΔH0 is not expected from the ion-association theories [8,9]. The solvent 0 composition dependence of H values for NiSO4, MnSO4 and ZnSO4 is shown in Fig. 2. It was shown that H0 values of all the electrolytes were decreased with increase of ethanol content in the solvent mixtures. Ion-ion interaction in solution might be responsible for these positive values of H0. The calculated H 0 values are found to follow the order: ZnSO4 >MnSO4 >NiSO4. 90 Islam and Mahmud The standard Gibbs energy changes (ΔG0) for the ion association process were calculated from the values of association constants using the following equation: 0 G RT ln KA It is well known that the values of association constant of pure electrolyte are mainly influenced by the radius of the electrolyte and the dielectric constant of the solvent medium. On the other hand, there is significant evidence that some ions are solvated in the solvent medium [10], i.e., each cation/anion is surrounded by a number of solvent molecules that escort the ions in solution. As a result, the values of the association constant also depend on the non-electrostatic interaction between the electrolyte and the solvent medium. The measured values of ΔG0 for all the electrolytes were found negative at various solvent compositions and are recorded in Table 1. It is shown that the negative ΔG0 values increase with increase of organic portion in the solvent mixtures for each electrolyte. These findings indicate that the released solvent molecules transfer into the bulk solvent and leads to smaller ΔG0 values. Fig. 3 represents the corresponding ΔG0 as a function of solvent composition of ethanol in the solvent mixtures at 250C. It demonstrates that the ΔG0 values of each electrolyte also decrease with decrease of dielectric constant of the solvent compositions. The figure shows almost linear plots except for the values in the 20 wt.% of ethanol. The negative G0 values illustrate that the ion-association process is spontaneous and the magnitude of G0 is a measure of the steady ion-pair formation. The calculated G 0 values are found to follow the order: NiSO4 >MnSO4 >ZnSO4. Table-1. The changes of standard Gibbs energy (G0) enthalpy (ΔH0) and entropy 0 (ΔS 298) values of NiSO4, MnSO4 and ZnSO4 in ethanol-water mixtures 0 -1 0 -1 0 -1 -1 Wt.% ΔG 298 in k J mol ΔH in k J mol ΔS 298 in J mol K NiSO4 MnSO4 ZnSO4 NiSO4 MnSO4 ZnSO4 NiSO4 MnSO4 ZnSO4 5 -13.81 -13.59 -13.42 15.55 16.64 17.06 98.53 101.45 102.30 10 -14.58 -14.07 -13.97 11.02 11.76 12.47 85.89 86.68 88.70 15 -15.01 -14.53 -14.38 8.38 9.51 10.17 78.49 80.66 83.06 20 -15.65 -15.59 -15.48 3.93 4.40 5.48 65.70 67.08 70.32 Thermodynamic Studies of Some Symmetrical Electrolyte’s Solution 91 In order to calculate the standard entropy changes (ΔS0) of the ion- association process, we use the following equation H 0 G0 S 0 T 0 0 Figure 2: Plots of H vs. wt.% of ethanol for NiSO4, MnSO4 and ZnSO4 at 25 C. In Table 1, the calculated ΔS0 values of the three electrolytes are recorded. It shows that all ΔS0 values are positive and decrease with increase of organic content in the solvent mixtures. These positive values of S0 along with H0 values indicate that the ion-association process is mainly entropy controlled. The dependence of ΔS0 on solvent compositions are illustrate in Fig. 4. It also shows that the ΔS0 values of all the three electrolytes decrease with decrease of dielectric constant values of the solvent mixtures which point out a deteriorating in the ion solvation. The positive ΔS0 values of all the three electrolytes may be attributed to the increasing number of degrees of freedom upon association generally due to the release of solvent molecules. In other words, the solvation of ions is weakened as a result of 0 the formation of ion pair. The ΔS values of NiSO4, MnSO4 and ZnSO4 in the all solvent compositions used in this study were found positive because of the decrease in solvation of the ion pairs compared to that of the free ions in the solvent mixtures [11,12]. The ΔS° values of the electrolytes are found to decrease in the following order: ZnSO4 >MnSO4 >NiSO4, which indicates that, the degree of disordering of the ion solvation due to the 92 Islam and Mahmud formation of ion pair also decreases in the same order.